The use of lignocellulose and carbohydrate-containing biomass, in general as a renewable feedstock for the synthesis of fuels, chemicals and materials, is promising as (lingo)cellulose and, thus, carbohydrate biomass, and is Earth's most abundant source of organic carbon. In light of the high prices and future lower availability of especially fossil-derived crude oil and all its derivative fractions such as light naphtha, diesels and jet fuels, efficient processes to convert carbohydrate-containing biomass into these fractions are highly desired. Although natural gas is still largely available, this mixture of short alkanes (containing less than 4 carbon atoms) is not easily converted into a light naphtha fraction without going through the expensive and highly energy-demanding combination of syngas production and Fischer-Tropsch processing of the derived syngas. Renewable and efficient processes for the conversion of carbohydrate-containing biomass into hydrocarbon rich fractions that are now obtained from crude oil are thus highly desired.
Mild catalytic processing of carbohydrate-containing biomass could be a promising method to transform this highly oxygen-functionalized feedstock into mixtures of alkanes, such as light naphtha, diesels and others fuels that are lean in oxygen atoms and contain a high number of combustible C—H bonds. With respect to pyrolysis and gasification, catalytic processes are more selective and demand less stringent conditions; whereas, with respect to enzymatic and biological processes, catalytic processes are faster and usually operate at higher productivities. The latter is needed in this context because the economic value of a fuel or alkane fraction per weight is usually low, but the targeted volumes and markets are huge. Process and energy cost for their synthesis should be kept to an absolute minimum.
Producing fuels and alkanes from biomass feedstock via catalytic processing primarily concentrates on sequential depolymerization of polysaccharides (if a polymeric feedstock is used), the formation of intermediates, the coupling of such intermediates and finally the deoxygenation of such intermediates or coupled intermediates to produce molecules with high heating value like alkanes and aromatics. There are elaborate examples in literature describing the production of new biofuels with often unique chemical structures that are, in fact, C—C coupled and (partially) deoxygenated chemicals made from biomass-derived sugars, sugar alcohols or other carbohydrate-biomass-derived platform molecules such as HMF, levulinic acid, angelica lactone, etc. A textbook example is the (cross-)coupling of 5-HMF with acetone or itself before deoxygenation to C7 to C15 range of alkanes with an expensive Pt/SiO2—Al2O3 catalyst (Huber et al., Science 2005 vol. 308, pp. 1446-1450). Another recent example is the deoxygenation of the angelica lactone dimer into a similar range of diesel-like products with a two-rare-element-containing Ir—ReOx/SiO2 catalyst (Mascal et al., Angew. Chem. Int. Ed. 2014, vol. 53, p. 1854). Again, another example is the synthesis of biofuel dimethylfuran from fructose (the isomer of glucose) using CuRu on carbon catalyst (Roman-Leshkov et al., Nature vol. 447 pp. 982-985). An overview of current biofuel technologies can be found in the following reviews: Huber et al., Chem. Rev. 2007 vol. 106 pp. 4044-4098; and Climent et al., Green Chem. 2014, vol. 1 pp. 516-547.
A related state-of-the-art technology is the aqueous phase-reforming (APR) process in which the oxygen content of carbohydrates and the feedstocks derived therefrom is reduced with in-situ-generated H2, so that the products are (after final hydrotreating) hydrocarbons. This APR is based on work originally described by Huber, Cortright, and Dumesic in (Huber et al., Science 2005 vol. 308 pp. 1446-1450; and Huber et al., Angew. Chem. Int. Ed. 2004 Vol. 43 pp. 1549-1551). APR is now being commercially developed by Virent, Inc., and often acid- or base-catalyzed coupling of intermediates is also applied to obtain higher carbon numbers in the product before deoxygenation (Bauldreay et al., U.S. 2013/0055626 A1; Cortright et al., U.S. Pat. No. 8,455,705 B2; and related or affiliated patents such as U.S. Pat. No. 6,953,873 B2 and WO2007075370). Such processes have the advantage that the product hydrocarbons are already well integrated as fuels in existing automotive and engine infrastructure. The major disadvantage is that the method operates only on monosaccharide or disaccharide sugars or starch, but the examples are not directly on cellulose or cellulosic biomass. Moreover, a complex metal catalyst is required and the overall yield of hydrocarbon products is medium to low as substantial amounts of carbon are lost in the reforming process (e.g., in the form of CO2).
Besides these coupling and/or deoxygenation strategies toward partially deoxygenated molecules or higher (C6+) alkanes, the production of C1-C6 alkanes with main products n-hexane and n-pentane has been reported directly from sorbitol (the hydrogenation product of glucose) either with external H2 or in-situ-produced hydrogen by simultaneous APR of sorbitol over a Pt catalyst (Huber et al., Angew. Chem. Int. Ed. 2004 Vol. 43 pp. 1549-1551). The reaction is not selective toward a high C5+ alkane yield, which should be the main economic target. Chen et al. (Chem. Sus. Chem. 2013 April; 6(4):613-21) demonstrated a process that uses a combination of Ir—ReOx/SiO2 and H-ZSM-5 mainly to selectively convert sorbitol to n-hexane via hydrogenolysis (and other lower sugar alcohols to lower alkanes). It was also demonstrated for glucose, but long reaction times were needed (up to 84 hours) due to the lower reaction temperature. The latter presumably cannot be higher due to the acceleration of C—C cracking reactions at higher temperatures. Back in 1994, Robinson filed a process for producing hydrocarbon fuels and mainly hexane from cellulose or hemicellulose (U.S. Pat. No. 5,516,960, 1996), but in essence, it comprised a multistep process beginning with the hydrolytic hydrogenation of the polysaccharide to the respective sugar alcohols, a process long since known in the art (Sharkov et al., Angew. Chem. Int. Ed. Engl. vol. 2: pp. 405-409), followed by subjecting the sorbitol (or other sugar alcohols) to a treatment in hydroiodic acid (HI) in boiling water to produce iodo-alkanes and hydrocarbons. Lastly, a method to convert methylated cellulose into alkanes with hydrosilylative deoxygenation via B(C6F5)3 in the presence of an expensive hydrogen source, like Et2SiH2, was reported; but again, a minimum of two steps is needed to methylate the cellulose. Although metal-free, the use of excessive amounts of expensive silane and B(C6F5)3 co-reagents limits its industrial prospects.
The one-step conversion of especially cellulose into to light naphtha or, e.g., n-hexane, is thus new, since both APR as well as derived processes or the HI-based process are multistep processes and/or only operate on sugars, sugar alcohols or sugar derivatives like the methylated cellulose. It would, however, be tremendously advantageous if raw polymeric cellulose or even lignocellulosic feedstock could be processed into a light naphtha fraction and mainly n-hexane right away. Due to the large natural abundance of cellulose and its uniform chemical structure with repeating C6 sugar units, cellulose is the ideal precursor for C6 alkane (or light naphtha) synthesis as C—C bond breaking and forming are not required. The biggest challenge is to selectively break C—O bonds in presence of C—C bonds and to avoid the formation of sorbitol. The sorbitol-to-alkanes route does not seem compatible with the depolymerization of cellulose.
Major obstacles for direct cellulose conversion are the poor solubility in conventional solvents and high recalcitrance toward chemical reactions. These drawbacks necessitate severe reaction conditions in terms of acidity and/or temperature, which could lead to unwanted side reactions, next to the already complex nature of the reaction network if n-hexane and light naphtha in general is targeted. A careful process incorporating a specifically modified hydrogenation catalyst is hereto proposed to accelerate only desired reactions toward formation of light naphtha.
An overview of this disclosure, which is capable of transforming cellulose directly into light naphtha via hydrodeoxygenation of in-situ-formed 5-HMF is found in FIG. 1 and the differences with the state of the art are partially pointed out in FIG. 2.